1. Field of the Invention
This invention relates generally to the field of chemical vapor deposition (CVD) reactors for thin film deposition, especially of epitaxial films, and more particularly to CVD reactors employing one or more lamp-heated reactors and a travelling wafer sleeve exposed to the lamps, absorbing the lamp radiation, and mounting multiple wafers and defining process gas flow within the wafer sleeve.
2. Description of the Related Art
Epitaxial reactors for use in depositing thin films on wafers by chemical vapor deposition (CVD) may be categorized in terms of their method of heating the wafers, their overall arrangement of the wafers within the reaction chamber or chambers, and the overall tool architecture, including the number of reaction chambers and whether additional chambers for preheat and cool down are configured at the entrance and exit of the system, respectively. FIGS. 1-3 illustrate three different types of prior art epitaxial reactors, categorizing each in terms of these various design aspects.
A prior art pancake-type epitaxial reactor 100 is illustrated in the schematic side cross-sectional view of FIG. 1. The wafers 110 on which epitaxial films are to be deposited are supported by a susceptor 111. Typical susceptors may be composed of graphite with a silicon carbide coating. The susceptor 111 is mounted within a reactor chamber 101 into which one or more process gases 102 enter through an inlet line 103 to a gas passageway included within a stem which also provides mechanical support for rotary motion 140 of the susceptor 111. Electrical eddy currents flowing within the resistive graphite material of the susceptor 111 heat the susceptor 111 and, by conduction, the wafers 110 supported thereon. These eddy currents are induced by a set of RF induction coils 112 mounted beneath the susceptors 111. Process gases 105 enter the reactor chamber 101 through an outlet 104 from the gas passageway in the stem and then flow across the surface of the heated wafers 110. Exhaust gases 115, comprising both product gases from the epitaxial reaction as well as unused reactant gases, are pumped out of the reactor chamber 101 through outlet openings 114.
Pancake-type epitaxial reactors 100 have the ability to deposit thick films and dual layers with non-uniformities in the range of 4% in thickness and 7% in resistivity with sharp transitions and low metals contamination. The rotary motion 140 of the susceptor 111 enhances deposition uniformity. Key disadvantages of this type of epitaxial reactor are low throughput, high gas consumption, wafer warpage, and worse uniformities than other types of prior art epitaxial reactors (see FIGS. 2 and 3). Another important disadvantage is the need for frequent cleaning of the inner surfaces of the reactor chamber 101 due to unwanted deposition of films on these surfaces. This unwanted deposition increases the cost of ownership due to higher process gas consumption and increased system downtime for maintenance and cleaning.
A prior art barrel type epitaxial reactor 200 is shown in the schematic side cross-sectional view of FIG. 2. In this type of reactor, wafers 210 on which epitaxial films are to be grown are mounted on a multi-sided graphite carrier 211, which is held within a reactor chamber 201 on a support 215 enabling rotary motion 240 to enhance uniformity during the deposition process. The graphite carrier 211 is heated by an array of lamps 202 contained within one or more reflector assemblies 203 which are mounted around, and outside of, the reactor chamber 201. Process gases 217 enter the reactor through inlet lines 216 and flow around the outside of the graphite carrier 211 as illustrated by arrows 220. Exhaust gases 205, comprising both product gases from the epitaxial reaction as well as unused reactant gases, are pumped out through an exhaust line 204. Some of the reactant gases 221 recirculate within the reactor chamber 201, increasing the usage efficiency of the reactant gases during the epitaxial deposition process.
Barrel-type epitaxial reactors have the advantages of good surface quality and slip performance, with typical thickness non-uniformities around 3% and resistivity non-uniformities around 4%. Throughputs can be higher than for the pancake type reactor. Some disadvantages are an inability to deposit dual layers and relatively high film resistivities. This type of reactor is currently the main type used in CMOS semiconductor manufacturing.
A third type of prior art epitaxial reactor is illustrated in FIG. 3, a single/mini-batch lamp heated reactor 300. In this reactor 300, a single wafer 310 is processed within a reactor chamber comprising a lower metal portion 301 and a quartz dome 302, which are held together by a multiplicity of clamps 303. Typically, the pressures within the reactor chamber may be lower than for other types of epitaxial reactors. A wafer 310 being processed is supported by a carrier 311 which is mounted on a stem 315 extending into the reactor chamber. The stem 315 enables rotary motion 340 of the carrier 311 during the deposition process to enhance uniformity. Process gases 307 enter the reactor through an inlet line 350. The process gases 308 enter the reactor chamber near the level of the wafer 310. The wafer 310 is heated by light radiation from an array of lamps 330 mounted within a reflector assembly 331. Exhaust gases 306, comprising both product gases from the epitaxial reaction as well as unused reactant gases, are pumped out of the reactor chamber through an outlet opening 305.
The single/mini-batch type of epitaxial reactor has a number of important advantages, including the ability to process wafers 310 with no need for a backside seal. Epitaxial films may be deposited with sharp dopant transitions, with low metals contamination, and with low thickness (1.5%) and resistivity (2%) non-uniformities. In addition, wafers up to 300 mm in diameter may be accommodated in the single wafer reactor chamber. Films with good surface quality and no slip may be deposited with throughputs as high as 8 to 10 wafers per hour. However, an important disadvantage of this type of reactor is the high film costs for thicker films where the throughputs drop due to the longer epitaxial deposition times required.
Epitaxial deposition is a process which was pioneered for use in the semiconductor industry for the manufacture of integrated circuits and discrete devices. Typically, a silicon-precursor gas such as silane is injected close to a hot crystalline silicon substrate to chemically vapor deposit a layer of silicon on the substrate, which is epitaxial with the silicon of the substrate. For these applications, in general, the final value of a fully processed wafer can be fairly high, in some cases, such as for microprocessors, in the tens of thousands of dollars per wafer. Thus, the economics of semiconductor manufacturing may support relatively higher costs for each processing step than would be the case for other types of semiconductor products such as photovoltaic (PV) solar cell wafers. For these other applications, the cost per process step must be relatively low since the final cost of a PV solar cell (typically about 150 mm square) may be in the range of ten dollars, orders of magnitude lower than for most fully processed semiconductor device wafers (typically 200 or 300 mm in diameter). On the other hand, some film characteristics for PV solar cell applications may be less stringent than for device wafers, in particular, the required film thickness and resistivity uniformities.
Epitaxial deposition of a thin-film solar cell has the disadvantage that epitaxial deposition is typically a relatively slow process in achieving good epitaxy but the semiconducting light absorbing layers in a solar cell need to be relatively thick. As a result, the deposition times for epitaxial solar cells are typically much longer than for the very thin epitaxial layers typical of modern electronic integrated circuits.